The present invention relates generally to magnetic field sensors and more specifically to magnetoresistors.
Modern motor vehicles are equipped with numerous sensor assemblies that are used to determine the relative position of a moving part, such as a steering column, to a stationary part, such as the vehicle chassis. These sensor assemblies can include a magnet for creating a magnetic field and a magnetoresistor (MR) for sensing the magnetic field and any changes thereto. It happens that indium-antimonide (InSb) MRs are extremely sensitive to magnetic fields. Unfortunately, they are also very temperature dependant. In order to compensate for the temperature dependency, an InSb MR can be incorporated into a voltage divider with another InSb resistor that has a similar temperature dependance, but does not have any magnetic field dependence. The second InSb resistor acts as a reference resistor.
It is advantageous to integrate both InSb resistors on the same substrate to create a single circuit chip. To double the voltage output and hence increase the signal strength, two of these voltage dividers can be connected together to make a Wheatstone bridge. A common to do this is to make the MR out of an InSb film patterned with magnetic shorting bars that short out the Hall voltage and create the geometric magnetoresistive effect. The reference resistor is made out of the same InSb film sans the shorting bars. Then, both InSb resistors are connected together to make a voltage divider. Two of these voltage dividers can then be connected together to create the Wheatstone bridge.
As recognized by the present invention, the above approach has two major disadvantages. First is that the reference resistor has an intrinsic magnetoresistance that, although being smaller than that of the geometric resistance, it is nonetheless significant and is temperature dependant. As a result, the temperature compensation effect of the MR/reference resistor pair becomes dependant on the magnetic field and does not work properly over a useful range of magnetic fields. The second major disadvantage is that the reference resistor takes up space on the semiconductor die and nearly doubles the area necessary to make the magnetic sensor assembly described above. Thus, the cost and size of the die is increased.
The present invention has recognized these prior art drawbacks, and has provided the below-disclosed solutions to one or more of the prior art deficiencies.
A magnetoresistor voltage divider includes a reference resistor and a sensing magnetoresistor. The sensing magnetoresistor is disposed on top of the reference resistor in a single circuit chip.
In a preferred embodiment, the magnetoresistor voltage divider includes a bottom layer, a middle layer disposed on the bottom layer, and a top layer disposed on the middle layer. Also, the magnetoresistor voltage divider includes a common terminal that electrically connects the bottom layer to the top layer. A reference resistor terminal is connected to the bottom layer; and a sensing MR terminal is connected to the top layer. Preferably, the reference resistor is established between the common terminal and the reference resistor terminal. The sensing magnetoresistor is established between the common terminal and the sensing terminal. In a preferred embodiment, the bottom layer and the top layer are resistive layers, and the middle layer is an insulating layer.
Preferably, the magnetoresistor voltage divider further includes plural top layer shorting bars disposed on the top layer. Also, in a preferred embodiment, the bottom layer is disposed on a substrate. Preferably, the bottom layer and the top layer are made from indium-antimonide (InSb) or indium-arsenide (InAs). On the other hand, the middle layer is made from one of: undoped gallium arsenide (GaAs), slightly p-doped gallium arsenide (GaAs), indium phosphide (InP), gallium antimodide (GaSb), indium gallium antimonide (InGaSb), and indium aluminum antimonide (InAlSb).
In another aspect of the present invention, a method for making a magnetoresistor voltage divider includes providing an substrate and disposing a bottom layer on the substrate. A middle layer is disposed on the bottom layer and a top layer is disposed on the middle layer. In this aspect, the substrate and the middle layer are made from an insulating material. The bottom layer and top layer are made from a resistive material. The method also includes connecting a reference resistor terminal to the bottom layer. A common terminal is connected to the middle layer and the top layer. Moreover, a sensing MR terminal is connected to the top layer.
In yet another aspect of the present invention, a magnetoresistor sensor includes a substrate and a bottom resistive layer on the substrate. The sensor also includes a top resistive layer that is spaced and insulated from the bottom layer and electrically connected thereto. A common terminal is electrically connected to the top and bottom layers. A sensing MR terminal is electrically connected to only the top layer. Also, a reference resistor terminal electrically connected to only the bottom layer. In this aspect, a sensing magnetoresistor is established between the common terminal and the sensing MR terminal. Moreover, a reference resistor is established between the common terminal and the reference resistor terminal.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: